I love clematis! I agree that the montana types are VERY hardy and scramble everywhere. They're great for covering things - garages, fences, whatever. Also, the Viticella types, which flower later in the summer, flower well and are less susceptible to clematis wilt. There are several Clematis Viticellas - smaller flowers than the early-summer flowerers, but just as pretty. Again great for growing through trellis, trees, with roses (you could get one to complement or contrast with a climbing rose) or a vine
The montana types are pruned after flowering - or just leave them to it, and give them a good hacking every few years or they could get into a bit of a birds' nesty tangle!
There is also a late-flowering clematis called Clematis tangutica - 'Bill Mackenzie' is a good variety. Otherwise known as Orange Peel clematis because their petals (ranging from different shades of yellow/gold/orange) curl like orange peel. These flower up till about October, when you can hack back all the top growth.
Whatever type you plant, plant it nice and deep - they like to be deep, and are more likely to succeed. Enjoy!